1. Field of the Invention
The present invention relates to a circuit for recharging a rechargeable battery, and in particular, to a battery recharging circuit capable of operating stably against a variation of a supply voltage.
2. Description of the Related Art
A common battery recharging circuit includes an AC-to-DC converter to convert an AC supply voltage into a DC charging voltage, and provides a rechargeable battery with the DC charging voltage. Further, in order to prevent overcharge of the rechargeable battery, the battery recharging circuit controls a supply of the charging voltage according to a charging status of the rechargeable battery.
Known battery recharging circuits can be widely divided into three types. A first type of known battery recharging circuit provides the rechargeable battery with a constant current regardless of a voltage level of the charging voltage. Since this prior art battery recharging circuit has a great power loss, there is a need of a separate solution for the heat radiation in the circuit. A second type of known battery recharging circuit has a temperature sensing function and an overcharge prevention function, to protect the circuit and the rechargeable battery from damages. Though this prior art battery recharging circuit is preferable, it is very costly and difficult to precisely control the circuit. A third type of known battery recharging circuit maintains a constant output voltage until an output current reaches a predetermined value, at which point the battery recharging circuit varies the output voltage level while maintaining the constant current value.
FIG. 1 is an example of the third type of known battery recharging circuits. As illustrated, a voltage converter 112, composed of a transformer, converts an AC supply voltage from a voltage source 111 into a charging voltage. A rectifier 113 rectifies and smooths the charging voltage output from voltage converter 112, and provides a rechargeable battery 100 with the rectified charging voltage output thereof. Voltage converter 112 and rectifier 113 constitute an AC-to-DC converter for converting the AC supply voltage into a DC voltage having the charging voltage level. A current sensor 114 senses a current of the charging voltage output from rectifier 113, and generates a control signal when the current exceeds a predetermined value. A switching circuit 115, composed of a photocoupler, is turned on and off in response to the control signal output from current sensor 114. A controller 116, composed of a switching element connected between voltage source 111 and voltage converter 112, controls the supply of the AC supply voltage to voltage converter 112 according to a switching status of switching circuit 115.
In operation, current sensor 114 compares the charging current of rechargeable battery 100 with a reference current and generates the control signal if the charging current is identical to or greater than the reference current. Then, switching circuit 115 and controller 116 operate to cut off the AC supply voltage being applied to voltage converter 112, in response to the control signal output from the current sensor 114. In this manner, the battery recharging circuit maintains the constant output voltage before the output current reaches the predetermined value, and varies the output voltage level while maintaining the constant output current, if the output current reaches the predetermined value. However, this prior art battery recharging circuit is very sensitive against the variation of the supply voltage. Thus, it is difficult to precisely control the constant charging voltage and current. Further, the prior art device has a great loss of the charging voltage, which results in an obstacle to minimization of the device.
Other known switched, or triggered, charging circuits are exemplified by U.S. Pat. No. 3,586,955 to Edmund Kisiel entitled Battery Charger Using A Controlled SCR To Provide Tapering Charging Characteristics and U.S. Pat. No. 3,688,177 to John B. Reeves et al. entitled Battery Charger. In Kisiel the battery voltage is compared to an independent reference voltage and a silicon-controlled rectifier is gated according to the comparison result. In Reeves et al. both the charging current and the battery terminal voltage are sampled to control the conduction duration of a controllable rectifier to change the battery charging current.